The electrolytic production of chlorine and caustic by the electrolysis of brine has been well known for many years. Historically, diaphragm cells using a hydraulically-permeable asbestos diaphragm, vacuum-deposited onto foraminous steel cathodes, have been widely commercialized. Such diaphragm cells, employing permeable diaphragms, produce NaCl-containing caustic which generally requires a de-salting process to obtain a low-salt caustic for industrial purposes.
In recent years, the chlor-alkali industry has focused much of its attention on developing membrane cells to produce low-salt or salt-free caustic in order to improve quality and avoid the costly de-salting processes. Membranes have been developed for that purpose which are substantially hydraulically-impermeable, but which will permit hydrated Na.sup.+ ions to be transported from the anolyte portion to the catholyte portions, while substantially preventing transport of Cl.sup.- ions. Such cells are operated by flowing a brine solution into the anolyte portion and by providing salt-free water to the catholyte portion to serve as the caustic medium. Caustic is produced at the cathode, and chlorine is evolved at the anode, regardless of whether a membrane cell or a diaphragm cell is employed.
As early as 1918, various patents have suggested the flow of electrolytes from one cell to another, in sequence. For instance U.S. Pat. No. 1,284,618 teaches and claims an apparatus wherein the catholyte liquor flows from cell to cell, gaining in caustic strength in each succeeding cell. By so doing, the average caustic concentration across all the cells is less than in the final cell; this permits greater caustic efficiency throughout the cells. The patent also teaches that the anolyte may also flow from cell to cell, either in the same direction as the catholyte series flow or in the opposite direction. The patent teaches that there is some percolation of cell liquor through the diaphragm, but postulates that the catholyte series flow would be even more advantageous if the diaphragm was impervious to hydraulic flow between the anolyte and catholyte. According to the patent, it is immaterial whether or not the anolyte is fed separately or in parallel, or fed in series with the catholyte. The patent teaches that the "spent" anolyte from the final cell of a series can be fed to the catholyte portion to serve as the catholyte liquor in which the concentration of caustic is incrementally increased through the series flow. The "spent" anolyte, however, is known to still contain a substantial amount of salt.
It is well known in the relevant arts that caustic efficiency depends on, and is generally inversely related to, the caustic concentration of the catholyte in membrane cells and diaphragm cells. It has been reported (44th Annual Conference, Water Pollution Control Federation, San Francisco, Calif., Oct. 3-8, 1971, page 12--paper by S. A. Michalek, et al, Ionics, Inc.) that caustic efficiency does not substantially depend on the salt concentration (salt utilization) of the anolyte. It is also reported there that the membrane employed was "an XR cation-transfer membrane" and that the anode was a "DSA" anode supplied by Electrode Corporation. It is believed that "an XR cation-transfer membrane" refers to Nafion.RTM. fluoropolymer membrane developed by E. I. duPont de Nemours as an electrolytic membrane and that "DSA" refers to a dimensionally-stable anode comprising a titanium substrate coated with a layer of ruthenium oxide. The article discloses (page 9) that "--the most economical and practical design was a simple two compartment membrane cell with independent water feed to the cathode." The cell is used in electrolyzing aqueous NaCl to produce H.sub.2 and NaOH at the cathode and Cl.sub.2 at the anode; then the so-formed NaOH and Cl.sub.2 is reacted to make sodium hypochlorite which is used in sewage treatment.
It is an object of the present invention to produce a highly pure aqueous caustic solution by the electrolysis of alkali metal halide.
Another object is to provide a process whereby the overall efficiency of a chlor-alkali electrolytic membrane cell, or bank of cells, is improved.
A further object is to provide a process whereby the alkali metal chloride in the anolyte of a chlor-alkali electrolytic cell is more efficiently used without a significant loss of caustic efficiency.
Still another object is to provide an electrolytic cell which is capable of operating for extended periods of time without suffering a substantial loss of current efficiency or undergoing a rapid rate of wear.